1. Field of the Invention
This invention relates to lean burn engines in general and more particularly to controlling the torque in a stratified charge direct injected spark ignited engine having a Lean NOx trap, LNT, in the exhaust system.
2. Prior Art
The ability of lean burn engines to meet emission regulations, in particular with respect to the oxides of nitrogen, NOx, is limited unless advanced exhaust aftertreatment devices are used. For NOx reduction the current state of the art technology points towards the use of a Lean NOx Trap, LNT, as a most practical solution.
However, the LNT has to be periodically purged to maintain its level of conversion efficiency. As an example, for direct injected spark ignited, DISI, engines running in the stratified mode, the LNT is typically purged by running the engine slightly rich of stoicheometry for a few seconds in approximately every fifty seconds.
One of the disadvantages encountered in doing the purge during operation of a running vehicle is that if the purge cycle is not properly controlled, an undesirable engine torque disturbance is created. This undesirable engine torque disturbance will be noticed by the vehicle driver resulting in unacceptable drivability conditions of the vehicle.
In some prior art solutions, an electronic throttle control system is used to control the generation of this undesirable engine torque disturbance. The electronic throttle control operates to adjust the airflow rate into the engine depending upon the desired levels of torque.
It is therefore an advantage of the present invention to control the purging of the LNT to avoid any undesirable engine torque disturbances.
It is another advantage of the present invention to utilize a supplemental torque apparatus operatively connected to the engine to either generate needed torque or to absorb excess torque.
These and other advantages will become evident from the following method for torque control for direct injected engines having a Lean NOx Trap, LNT, in the exhaust system and a supplemental torque apparatus operatively connected to the engine. The method first identifies the time for purging the LNT from an algorithm contained in the electronic control unit of the vehicle. The state of charge of the vehicle battery is monitored to determine the level of the battery charge. The algorithm then proceeds to change the operational mode of the engine from lean to a rich air/fuel ratio, wherein such change results in an increased engine torque.
During the rich air/fuel ratio mode, the increased level of carbon monoxide (CO) ratio purges the LNT. The algorithm controls and maintains the desired torque output of the engine during the transition from lean A/F to rich A/F and the rich A/F purge. Excess engine torque is converted by the supplemental torque apparatus into electrical energy that is stored in the battery. After a predetermined time the algorithm operates to change the operational mode of the engine from the rich air/fuel ratio to the lean air/fuel ratio. During the lean air/fuel ratio mode, the algorithm controls the release of the stored electrical energy.